Forward declaration fails with immer::map when it used to work

The following code:
```cpp
#include <immer/map.hpp>

struct Foo;

struct Bar {
    immer::map<int, Foo> m;
};
```
Fails to compile on master. It worked in 0.8.1.

https://godbolt.org/z/9onYa383r

Is this expected?